Recovering and Applying Potassium Dichromate from Sulphochromic Waste

Abstract

Abstract:A simple procedure for recovering potassium dichromate (K 2 Cr 2 O 7 ) from treatment of residual sulphochromic solution was proposed in the present work by means of cooling crystallization. The decrease of temperature favored the crystallization of K 2 Cr 2 O 7 due to the decrease of solubility. 5.0 L of sulphochromic wastes containing 48.08 g L -1 of Cr(VI) were treated and the process of crystallization was performed in three steps until crystals were not formed anymore. On each step the content of Cr total was determined by flame atomic absorption spectrometry and Cr(VI) by colorimetric method with 1,5-diphenylcarbazide, resulting in the removal of 91% and 92% of Cr total and Cr(VI), respectively. After the last step, the remaining Cr(VI) in the solution was reduced to Cr(III) from the addition of NaHSO 3 , recovering via precipitation in pH 8 approximately 36.13 g of Cr(OH) 3 . The final supernatant was discarded since chromium content was below the maximum limit established by the Brazilian legislation for effluents discharge, which corresponds to 0.10 and 1.0 mg L -1 of Cr(VI) and Cr(III), respectively. 628.4 g of K 2 Cr 2 O 7 were recovered and the salt was characterized by X-ray diffraction and differential thermal analysis. Its applicability was compared to the standard K 2 Cr 2 O 7 when determining the soil organic matter, in which there was no significant difference, thus inferring that the recovered compound may be incorporated on routine analyses. This recovering process allowed the reuse of K 2 Cr 2 O 7 , thus reducing costs with the acquisition of new reagents and environmental impacts caused by the inadequate discard of sulphochromic solutions.Keywords: Chemical waste; cooling crystallization; speciation; soil organic matter. ResumoUm procedimento simples para a recuperação do dicromato de potássio (K 2 Cr 2 O 7 ) a partir do tratamento da solução residual de sulfocrômica foi proposta neste trabalho por meio do processo de cristalização por refrigeração. A diminuição da temperatura favoreceu a cristalização do K 2 Cr 2 O 7 devido à diminuição da solubilidade. Foram tratados 5,0 L de resíduo de sulfocrômica com concentração de 48,08 g L -1 de Cr(VI) e a cristalização foi realizada em três etapas até que não houvesse mais a formação dos cristais. Em cada etapa foram determinadas as concentrações de Cr total por espectrometria de absorção atômica de chama e Cr(VI) pelo método espectrofotométrico com 1,5-difenilcarbazida, o que resultou na remoção de 91% e 92% de Cr total e Cr(VI), respectivamente. Após a última etapa, o Cr(VI) remanescente na solução foi reduzido a Cr(III) pela adição de NaHSO 3 , recuperando via precipitação em pH 8 cerca de 36,13 g de Cr(OH) 3 . O sobrenadante final foi descartado, pois as concentrações de cromo estavam abaixo do limite máximo estabelecido pela legislação brasileira para lançamento de efluentes, que são de 0,10 e 1,0 mg L -1 de Cr(VI) e Cr(III), respectivamente. 628,4 g de K 2 Cr 2 O 7 foram recuperados e o sal caracterizado por difratometria de raio-X e...